1. Field of the Invention
The present invention relates to a sheet supplying device for performing a sheet supplying operation, and in particular relates to a sheet supplying device for supplying sheets stacked in a sheet cassette by using suction means for attracting a sheet and feeding the attracted sheet to a sheet feed path of an image forming apparatus such as, for example, a copying machine.
2. Description of the Related Art
In general, image forming apparatus such as copying machines employ a continuous photosensitive recording medium, such as a microcapsule sheet, and developer sheets which are used in combination with the photosensitive recording medium. Developer sheets are stacked in a sheet cassette and usually fed one at a time from the sheet cassette by a semicircular separator roller made of rubber. The separator roller and the developer sheets have coefficients of friction which are different from each other. The separator roller frictionally separates the uppermost developer sheet from the stack of developer sheets, and delivers the separated uppermost developer sheet to a certain predetermined position.
When the uppermost developer sheet is fed out of the sheet cassette, the color developer layer on this uppermost developer sheet rubs against the next developer sheet (the color developer layer faces downward), and the developer material tends to be partially peeled off from the developer sheet. Thus, an image formed on the developer sheet may be damaged where the developer layer has been removed, resulting in poor image quality.
Another sheet supplying device includes suction cups for attracting sheets with a negative pressure and for feeding the attracted sheet to a certain position, as disclosed in Japanese Patent Application Laid-Open No. 55(1980)-93744, for example. Another such suction system is disclosed in U.S. patent application No. 07/472,183 (JAO 26152), the disclosure of which is herein incorporated by reference.
A sheet supplying device has a mechanism for lifting and lowering one or more suction cups to separate and supply sheets from a sheet cassette. The known mechanism includes an elevator arm supporting suction cups and a sheet sensor. When a sheet supplying operation starts, the elevator arm is lowered until the sheet sensor detects the upper surface of the uppermost sheet of the sheet stack in the sheet cassette. When the sheet sensor detects the uppermost sheet, the elevator arm is stopped, and the suction cups are actuated to attract the sheet using a negative pressure. In this mechanism, as the number of sheets remaining in the sheet cassette is reduced, the elevator arm must be lowered a greater distance.
Therefore, so that the elevator arm having the suction cup is capable of moving upwardly and downwardly over a greater distance, the conventional mechanism is relatively complex and requires a large amount of energy for changing the rotation of the driving motor to achieve the requisite vertical movement.